1. Field of the Invention
The present invention relates to a high-frequency electric field coupler used in a contactless communication system propagating high-frequency signals in a wide band utilizing electric field coupling and relates to a communication system and communication apparatus utilizing the coupler. More particularly, the invention relates to a high-frequency electric field coupler with excellent characteristics having a coupling electrode and a resonance unit operating to increase the amplitude of electrical charge accumulated at the coupling electrode at a predetermined resonance frequency, and the invention relates to a communication system and a communication apparatus utilizing the coupler.
2. Description of the Related Art
Most radio communication systems according to the related art such as radio LANs (local area networks) which are typically in compliance with IEEE802.11 and Bluetooth (a registered trademark) communication systems are electrical wave communication systems which propagate a signal utilizing a radiation electric field generated when a current is passed through an aerial (antenna). A communication system of this type has a problem in that a transmitting end of the system can be a source of an electric wave disturbing an adjacent communication system because an electric wave may be radiated regardless of whether there is a party to communicate.
Methods of radio communication other than electric wave communication as described above include contactless communication methods which utilize an electrostatic field or induction field. For example, existing contactless communication systems primarily used for RFID (radio frequency identification) utilize the electric field coupling method or magnetic field coupling method. An electrostatic field undergoes an abrupt attenuation inversely proportionate to the third power of the distance of the same from the source, and an induction field undergoes an abrupt attenuation inversely proportionate to the second power of the distance of the same from the source. Therefore, such a field allows radio communication to be performed at such an extremely low power that the intensity of an electrical field (the intensity of an electric wave) at a distance of 3 meters from a radio facility equals or falls below a predetermined level. No license is required to run a radio station in such a case. In the case of a contactless communication system of this type, no coupling relationship occurs and no electric field is radiated when the system has no party to communicate in the neighborhood of the same. Therefore, the system will not disturb any other communication system. Even when there is an incoming electric wave from a remote source, an electric field coupler of the system will not receive the electric wave. Thus, the system will not be interfered by any other communication system.
There are proposals on RFID tag systems including a set of communication aiding bodies which are disposed such that an RFID tag is located between a plurality of communication aiding bodies. RFID tags attached to a plurality of commodities are disposed such that they are sandwiched by communication aiding bodies. Thus, information can be reliably read from the RFID tags or written in the tags even when the tags overlap each other (See JP-A-2006-60283 (Patent Document 1) for example).